The present invention relates to a demodulator for facsimile equipment and particularly to a MODEM for facsimile equipment incorporating an automatic equalizer.
Generally, in the case of performing a high speed data transmission through a transmission line (circuit) of a limited frequency band, received data are influenced by various deterioration factors, e.g. attenuation distortion and delay distortion. In this state, therefore, it is difficult for a receiving section to effect data identification. In order to correct the above distortions, an equalizer is provided within a MODEM.
Since the circuit distortions are not constant but vary with the lapse of time, it is required to maintain an optimum state of reception by following variations in circuit characteristics and correcting equalizer coefficients. Particularly, in a polyphase, multi-level modulation system, the interval between transmission data is so narrow that the deterioration factors on the transmission line have a great influence. Therefore, the demand for maintaining an optimum receiving state is strong.
In this connection, there is used an automatic equalizer for obtaining an optimum state of reception by performing an arithmetic processing in a real-time manner at every time slot, changing equalizer coefficients according t the result of the calculation and correcting a received signal.
In a demodulator for facsimile equipment there is used a complex type MSE method equalizer as an equalizer conforming to the CCITT Recommendation T.4 modulation system which is a polyphase multi-level modulation system
In a conventional complex type MSE method automatic equalizer, when the deterioration of circuit characteristics is slight and there is little error between equalizer output data and signal spot arrangement, equalizer coefficients are corrected exactly and there is performed an automatic equalization. But in the case of a large error caused by a large change in transmission characteristics for example, there will not be made an exact correction, but rather the correction will be made in an undesirable direction. If the error is an instantaneous error, a correctly modified value will finally be obtained through successive modifications. On the other hand, in the event a large erro continues to exist for a relatively long period, there will not be obtained an exact correction value. This state is called divergence or equalization error.
In facsimile equipment, there is provided, for a certain period, a training check signal (TCF) for placing an automatic equalizer in an operable state prior to the normal data transmission according to the binary system procedure defined by CCITT Recommendation T.30, to thereby set equalizer coefficients of the automatic equalizer, and thereafter a picture data message is provided. Therefore, in the event of divergence or equalization error caused by some line trouble, this will result in loss of picture data even under progress of the procedure in facsimile equipment. In the conventional demodulator having an automatic equalizer, there is no method to cope with such situation and it is has been necessary to start the transmission over again.
As to the correction of equalizer coefficients in a conventional complex type MSE method automatic equalizer, such is described in Japanese Patent Laid-Open Publication No. 109434/82.